1. Field of the Invention
The present invention relates generally to an inverter-drive controlling apparatus, and particularly concerns the inverter-drive controlling apparatus which is especially suitable for drive-controlling operation of an induction motor of a relatively small output power for industrial use such as for a compressor of air conditioner, or refrigerator, or the like.
There are several known types of control apparatus for inverter to drive a motor, such as of PAM, PWM type. Among them, PWM of inequal width simulated sinusoidal wave is superior in power source utility, miniaturization and light weight of apparatus, low noise of electromagnetic wave, low mechanical noise, low vibration, etc., and becomes major trend in recent years.
The PWM of simulated sinusoidal wave is that which is, as shown in FIG. 3 and FIG. 5, a system to produce PWM algorithm in a manner to simulate the sinusoidal wave with integral value of pulse voltage fed to motor winding.
Now, a prior art of HALT system, which is a basis to make the present invention, is elucidated as the prior art, with reference to FIG. 1, FIG. 2, FIG. 3, FIG. 4, FIG. 5, FIG. 6, FIG. 7, FIG. 8, FIG. 9, FIG. 10, FIG. 11 and FIG. 12. In FIG. 1, alternate current frcm a commercial power source E is rectified and smoothed by a rectifier-smoother 1, and the rectified and smoothed DC output from the rectifier-smoother 1 is given to an inverter 2, and the output of the inverter 2 is fed to an electric motor 3, and an inverter drive controlling circuit 4 gives the inverter 2 a controlling signal.
Nextly, one example of the general inverter system constituted for an air conditioner is shown in FIG. 2.
In FIG. 2, numerals 1, 2, 3 and 4 designate the rectifier smoother 1, inverter 2, electric motor 3 and inverter drive controlling circuit 4 of FIG. 1, respectively, and the inverter drive controlling circuit comprises a PWM algorithm generator 4a and a base current driver for supplying base currents to the basses of transistors Tr.sub.1, Tr.sub.2, Tr.sub.3, Tr.sub.4, Tr.sub.5, Tr.sub.6 in the inverter 2 and a photo-coupler 4b which couples the PWM algorithm generator 4a and the base current driver 4c in insulated manner.
Signals generated by the PWM algorithm generator 4a are amplified and conveyed by the photo-coupler 4b and given to the base current driver 4c, and after current amplification the signals are given to the inverter 2. In the inverter 2, the transistor pairs Tr.sub.1 and Tr.sub.2, Tr.sub.3 and Tr.sub.4, and Tr.sub.5 and Tr.sub.6 each constitute inverter switches and either one of each transistor in the pair is selectively turned on at a time. Junction point between the transistor pairs are connected to three terminals UVW of the electric motor 3 of a compressor of an air conditioner.
FIG. 3 shows wave forms of signals to be applied to the bases of the transistors Tr.sub.1 through Tr.sub.6, and waveforms of voltages to be applied across the windings of electric motor 3. In FIG. 3, waveforms U, V and W correspond to the signals applied to the bases of the transistors Tr.sub.1, Tr.sub.3 and voltages to be applied across the windings of electric motor 3. In FIG. 3, waveforms U, V and W correspond to the signals applied to the bases of the transistors Tr.sub.1, Tr.sub.3 and Tr.sub.5. U--V, V--W and W--U are waveforms of voltages applied to respective windings of the electric motor 3.
As shown in FIG. 3, the waveforms of the voltages are designed to be simulated to sinusoidal wave when integrated, and period of the pattern of this voltage determines revolution number of the electric motor 3.
Now, PWM algorithm is elucidated with reference to FIG. 4 which elucidate concept of carrier. Half period of the sinusoidal wave of FIG. 4 is equally divided by an integer N. This integer N is called "carrier", and the small period T.sub.0 made by dividing the half period of the sinusoidal wave by the carrier N is called "carrier period". By issuing pulses in respective period T.sub.0 with pulse widths responding to voltage at that divided period T.sub.0 of the sinusoidal wave, the algorithm as shown by FIG. 3 is produced.
Nextly, voltage value to be applied to the coils of the electric motor 3 is elucidated with reference to As shown by FIG. 5(a), it is provided that pulses of a predetermined voltage and having pulse widths corresponding to a sinusoidal wave having a value of an integral of the pulses are generated by means of an algorithm. When the pulse widths of the pulses are increased in proportional way, the waveforms become as shown in FIG. 5(b), namely, the value of integral of the pulses increases. Accordingly, the amplitude of the sinusoidal wave can be controlled by changing of the pulse widths.
Nextly, relation between the pulse widths which defines the output voltage (amplitude of the sinusoidal wave) and HALT is elucidated with reference to FIG. 6(a) and FIG. 6(b). FIG. 6(a) shows a situation wherein the carrier period T.sub.0 comprises plurally divided times of region of data, and the HALT region si defined as the remaining time in the carrier period T.sub.0 such as T.sub.0 (i). It is defined that in this HALT region, no voltage data is output. Now, it is provided that time period of the data region is sufficiently smaller in comparison with the carrier period T.sub.0 (1), as shown in FIG. 6(a).
And nextly, it is provided that, as shown in FIG. 6(b), carrier period T.sub.0 is halved into T.sub.0 (2) from that of T.sub.0 (1), and that the time period of the data region is unchanged. Then, frequency f of the carrier becomes 2-times (since carrier period T.sub.0 (2)=1/2.multidot.T.sub.0 (1)), and output voltage is also doubled. This is because that the relative pulse widths with respect to the carrier period T.sub.0 (2) is 2-times of the pulse widths with respect to the carrier period T.sub.0 (1).
A small time unit is defined by dividing the time period of the data region DATA of FIG. 6(a) and FIG. 6(b) by an integer K, and this time unit is named as "data unit timer T.sub.2 ".
Then, by fixing the data unit timer T.sub.2 to a constant length and changing the carrier period T.sub.0, the frequency f is changed in an inverse proportion, and output voltage is changed in proportion to the frequency. Responding to the change, the HALT period, which is the period when no data is produced, also changes.
The above-mentioned frequency-output voltage relation is shown in FIG. 7.
Now, further detailed description is made with respect to the data region, with reference to FIG. 8(a) and FIG. 8(b). In these time charts, a sampled voltage is represented by the unit timers T.sub.2 of a number responding to the value of the sampled voltage, and therefore, the voltage is represented by a logic pattern having K resolution.
Naturally to say, when the carrier N and the integer K are selected as larger number, the waveform of the voltage to be applied to the electric motor is made more smoothly simulated sinusoidal wave.
As shown in FIG. 8(a) and FIG. 8(b), both cases have the same carrier period T.sub.0 (1), but the data unit timer T.sub.2 (1) of FIG. 8(a) is only half length of time of the data unit timer T.sub.2 (2) of FIG. 8(b). Accordingly, the data region time length T.sub.2 (2).times.K of FIG. 8(b) is 2-times of the data unit timer T.sub.2 (1).times.K of FIG. 8(a), and HALT time of FIG. 8(b) accordingly becomes smaller than the HALT time of FIG. 8(a). In these cases, the output voltage of FIG. 8(b) is 2-times the output voltage of FIG. 8(a). Accordingly, the frequency-voltage graph of FIG. 9 plotted with the data unit timers T.sub.2 (1) and T.sub.2 (2) as parameter becomes as shown in FIG. 9.
For a certain frequency, for instance, represented by a vertical line in FIG. 9, when voltage goes up the parameter changes from T.sub.2 (1) to T.sub.2 (2) and so on, and the HALT region decreases; and at extremity, the HALT region vanishes. For a certain rectified and smoothed DC voltage from the rectifier smoother 1, a maximum voltage to be impressed on the electric motor 3 is determined by this condition. Accordingly, even though the frequency is increased further from that condition, the voltage to be impressed on the electric motor 3 does not change further. The above-mentioned situation is elucidated with reference to FIG. 10(a) and FIG. 10(b).
As shown in FIG. 10(a), a carrier period T.sub.0 (3) is equally divided by an integer number K thereby defining the data unit timer T.sub.2 (1)=1/K.multidot.T.sub.0 (3), without retaining the HALT region at all. That is, the relation T.sub.0 (3)=K.times.T.sub.2 (1) holds. Then, provided that the frequency f is raised so as to have a shorter carrier period T.sub.0 (4) shown in FIG. 10(b) than a previous carrier period T.sub.0 (3), the data unit timer T.sub.2 (3) is given by an equation T.sub.0 (4)=K.times.T.sub.2 (3). At this frequency change, ratios of data region period against carrier period T.sub.0 are kept constant, and accordingly the voltage obtained from the both cases are equal each other as shown by FIG. 11.
Nextly, relation between the inverter output and load is elucidated. When the load is a resistance load, the inverter output is proportional to square of voltage. On the other hand, with respect to a compressor of an air conditioner, amount of work is proportional to exhaustion volume of refrigerant from cylinders of the compressor, and accordingly, the exhaustion volume is proportional to revolution number of the electric motor. Accordingly, it is preferable that frequency f and the output voltage should have a predetermined proportional relationship.
On the other hand, actual electric motor for the compressor shows effect of iron loss and copper loss, etc., and therefore, in low frequency range it is necessary that its driving voltage should be increased in order to compensate the above-mentioned iron loss, copper loss, etc. That is, boost function is necessary.
In the prior art apparatus, the boosted curve was realized by adding corrections by obtaining the carrier period T.sub.0 and data unit timer T.sub.2 by analog timer circuits, and the carrier frequency is set by means of the carrier period T.sub.0, and the compensation is added to the unit timer T.sub.2 responding to the set value of the carrier period T.sub.0. Greatest advantage of the HALT system is that by only changing the carrier period T.sub.0 and data unit timer T.sub.2, the PWM algorithm can be obtained for any frequency regions by providing only one period of algorithm generation pattern.
The above-mentioned prior art apparatus has the advantage that, when circuit to produce the carrier period T.sub.0 and the data unit timer T.sub.2 are realized by analog timer circuit, minute variation of the timer value can be made by handling circuit component of the external circuit, and the carrier period T.sub.0 and the data unit timer T.sub.2 can be adjusted independently each other. But the prior art has a problem that when frequencies to be used widely distribute and when close simulation to the sinusoidal wave is intended, there is a necessity that the carrier N and the integer K (number of data) should be switched. That is, in a low frequency range where resolution of the sinusoidal wave becomes rough and simulation of the sinusoidal wave becomes difficult, it is necessary that the carrier N and the integer K must be selected large. And on the other hand, when the carrier N and the integer K are large in a range of high frequency of f, switching speed of the transistors Tr.sub.1 through Tr.sub.6 becomes a great problem. That is, due to limit of the switching speed of the transistors Tr.sub.1 through Tr.sub.6, OFF-times of the transistors occupy high ratio in the operation, and therefore output voltage becomes low. Accordingly, the carrier N and the integer K must be limited to a reasonable small number.
In the prior art analog timer system, though PWM generation data itself relating to change of the carrier N and data number K can be made by external data area such as ROM or the like, smooth switchings between two kinds or more analog timers is difficult in view of transiential phenomena. For instance, in case that such switching is made by changing the carrier period T.sub.0 and data unit timer T.sub.2 with even a small difference, target frequency or target voltage happens suddently to change even at a short instance, and therefore the compressor may have an overcurrent or locking or at some instance, the power transistors will be damaged.
Furthermore, though above-mentioned prior art system has boosting function at the low frequency range as shown in FIG. 12, when the voltage of the power source is lowered the output voltage induced by the PWM also is lowered, thereby lowering torque of the compressor and increasing current of the motor, leading to further inducing of breakdown and efficiency drop of the compressor motor.